When Julian Opie needed an assistant he placed an advertisement in the Guardian. Within days, his immaculate studio had turned into an artistic version of the Blue Peter depot following an especially emotive appeal. Every surface was piled with CVs, Opie's voice mail and email boxes were jammed with keen inquiries, and then there were those so eager that they came by in person. Opie received more than 500 applications for a job which, if few outsiders are even aware exists, is increasingly integral to the contemporary art world.

'It was a crazy time,' admits Opie, who sat up with his wife every night for weeks trying to award marks out of 10 to each applicant. 'But my feeling is that a lot of other artists, also busy and stressed up to their eyeballs, might think that sort of professional approach was not such a bad investment.'

Indeed, Opie passed one of his applicants to his fellow Tate trustee, Turner Prize-winner Chris Ofili. He time-shares another assistant (he ended up with several) with fellow artists including Fiona Rae and Lisa Milroy. It's a fraught business: when one well-known artist needed a new assistant, word is that the member of her gallery staff charged with finding one was threatened with dismissal if unsuccessful.

So why all the fuss? Mostly it's to do with the pressures of fame and the fast pace of celebrity artist life. Like film stars they need a personal assistant to field media enquiries, run their diary and coordinate exhibitions round the world. In common with a film star's assistant, the role requires discretion, tact and extreme efficiency. The reward is proximity to fame and fortune, which can be full of career-building opportunities.

The other kind of artist's assistant - such as Tracey Emin's blanket stitcher Steven Gontarski - actually help to realise the work. Opie was hoping to combine the two roles in one person. His three by four centimetre advertisement, published on Monday 13 January, requested a PA studio manager with computer and office skills, plus technical, manual and painting fabrication abilities for £17-20,000 a year. Many applicants had none of these qualities, but were seduced by Opie's fame.

'I put my name in the advert and a lot of people expressed surprise that I'd made myself so available,' says Opie. 'Perhaps it was a little naïve, and I was overwhelmed by the number of emails that came addressed to me. But fame is a very strange quality, it's not that anyone recognises me, but the reaction did show me that my work is known among young people in this country.'

This would not surprise people who know Opie's cartoon-style portraits of the pop group Blur which adorned the cover of their recent Best of album. Or those worldwide who've seen his portraits of the Formula One team, or his work exhibited at Tates Modern and Britain, and at the opening of Baltic at Gateshead last year.

So Rebecca Partridge was overwhelmed when she was invited for her interview. 'I thought this was the most famous artist, more famous than any I've ever met, and afterwards I was like, "Wow, I've been in Julian Opie's studio."' Partridge, 26, from Yorkshire, studied painting in Bath. She worked in an art shop, painted theatre sets and was unemployed before getting the part-time job with Opie: she is now the only student from her college year currently working as an artist.

'Being an artist's assistant is an underground thing. I didn't know how it worked at all, that most artists have them. Julian's advert in the paper was like a door opening: you creep in and realise that they're people after all,' says Partridge.

'Julian is this professional married man who goes to work every day,' she adds. 'He's not like a tormented artist in his garret, and that really surprised me: he's a complete perfectionist which has really affected my work. I hope I'll combine an MA with doing this and then hopefully be employing my own assistant.'

Yet Partridge's own work, recently exhibited at Rhodes + Mann gallery in London's Hoxton, is unlike Opie's. Subsuming her style to his has made her pause: 'When I started working for Julian and I was just doing handmade paintings, even my most open-minded friends were asking big questions about the authorship of the paintings. They have to be absolutely flawless to the point where they look like prints, and that requires quite a bit of skill and energy. It's hard to remember that it's Julian's painting, not mine, and that it will be he who signs it.' Of course, it is Opie who designs and creates the template for the work.

Partridge could take comfort from the former artist's assistants who have gone on to bigger and better achievements. Opie himself worked for Michael Craig-Martin, the artist and Goldsmiths tutor. Opie's successors include Liam Gillick, nominated for last year's Turner Prize, Steven Gontarski and television filmmaker Mark James, who has made programmes about Damien Hirst.

'I think I must choose my assistants quite well,' muses Craig-Martin. 'My current assistant, Paul Hosking, was in last year's Becks Futures awards at the ICA. The trouble is that then they leave - that's what happened with Julian, he was successful too quickly to want to be anyone's assistant.' It was Craig-Martin who introduced Opie to his dealer, Nicholas Logsdail of the Lisson Gallery, who gave Opie his first solo show.

Fiona Rae only lasted a day as an artist's assistant. 'I stretched a couple of big canvases for Bruce McLean, probably not very well, and then he asked me to type a list out for him. He wanted it lower case and no punctuation, I thought he couldn't mean it, so put it all in. He found me another job and was very nice about it - even gave me a print.' Rae had more success with her own assistants, who she started using to stretch, prime and prepare canvases in 1991, the year she was nominated for the Turner Prize. She found love with one, Richard Patterson, who is now, like her, represented by Timothy Taylor Gallery.

In Damien Hirst's recent autobiography, he described Rachel Howard as his best ever spot painting assistant. This autumn Howard has her own solo shows at the Bohen Foundation in New York and at the Anne Faggionato gallery in London. Howard prefers not to comment on her days painting spots for Hirst, but her successor Lauren Child, now an acclaimed children's writer and illustrator, describes working for Hirst as 'an odd but brilliant job'.

Child and another part-time assistant painted coloured spots on to Hirst's famous canvases alone in a room in Borough, south London, in the late Nineties. 'I perhaps saw Damien Hirst three times in relation to what I was doing as a studio manager,' recalls Child. 'Pretty much all I did was paint spots. I have no idea how many spot paintings I did, we kept no record, but we were pretty quick. There's not that much to think about apart from how quickly you can do it, and the colours, which were meant to be random so we could choose them.'

Hirst is said to get a free helicopter ride courtesy of his dealer as a reward for large sales. Didn't this infuriate Child? 'No, artists have always had assistants - I admired and was inspired by his achievement. I didn't want to get involved in someone else's career and so it was perfect.'

The National Gallery's curator of Dutch seventeenth-century paintings, Axel Rüger, is intrigued by the parallels between today's artist's assistants and those of Rembrandt's day. 'Rembrandt ran very large workshops with pupils who had to pay for the privilege. And they'd work with the artists for many years. There's no way that one artist could have cranked out those hundreds of paintings, so they would work with assistants, and a master's crucial touches to painting were sometimes even contractually determined.'

Rüger argues: 'The workshop may no longer be technical, but if hundreds of people apply should Julian Opie advertise for an assistant, it suggests that the basis of the studio of the very famous artist has not changed, just the motives, and the artist's "name" remains every bit as important.'

The biggest name in twentieth-century art history, Andy Warhol, took inspiration from his assistants and turned the process into a happening in its own right with his Factory. So when Sarah Morris, who has a solo exhibition at White Cube in the spring, was studying in New York, she decided she wanted to work for Warhol's successor elect, Jeff Koons. 'Jeff made up a role for me. A lot of it was that I was just paid to be around. And he was producing a lot of stuff in Europe at the time, so it was slightly like Charlie's Angels : he'd be squawking instructions at me from a speaker phone.'

Morris uses assistants in every aspect of her painting and video work, inspired to delegate by Koons's example, and is even planning a joint exhibition with one of her assistants in Spain. She likens the studio system to that of Hollywood: 'It reminds me of Bob Evans's book and film, The Kid Stays in the Picture. There's far more infrastructure and collaboration going on inside the art world, a chain of command, than people see from the outside.'

Yet of course all is far from sweetness and light in the creative crucible of the artist's studio. And the six degrees of separation within the artist's assistants system - Opie used to work for his Goldsmiths tutor, artist Michael Craig-Martin, who employed last year's Turner Prize nominee Liam Gillick, who is married to Sarah Morris - means that, should there be a falling-out, repercussions can reverberate around the art world. Rachel Whiteread used to work for Alison Wilding: so apparently acrimonious was their parting that it's not a subject people choose to discuss.

Andrew Wilson, deputy editor of Art Monthly, says: 'As a historian it's always interesting to see how assistants develop in relationship to the artists they had once worked for. You get an affinity or a "kill the father" syndrome.' Wilson cites the example of Turner Prize nominees Jake and Dinos Chapman, who worked as assistants for Gilbert & George. The work of both pairs contains parallels in its use of shock, yet their public philosophical pronouncements are markedly at odds with each other.

'However, a classic example of a "kill the father" relationship, which over time reveals similarities and closeness, is that between Henry Moore and Anthony Caro,' argues Wilson. 'Caro worked for Moore, then his work was held up to be so much better and very different, yet has increasingly returned to the configuration of Moore over the past 15 years. Perhaps when you make the break between being a studio assistant and an artist in your own right, you have to distance yourself from the artist you once worked with.'

Mark Titchner worked for Keith Tyson, last year's Turner Prize winner, for two years and had a solo exhibition at Tate Britain's Art Now room this summer. 'My main contribution was the studio drawings. My style was fairly generic, and every day Keith would come in with a list and say what he wanted drawing. So my father, for example, found it odd that my work was in Keith's Turner Prize exhibition, but to me it was like working as a set painter. They were authorless drawings, that's the point of Keith's work, and I didn't think of them as mine at all. Using assistants is very much part of modern practice. Unless you're working only as a painter, you need other people for welding, picture editing, whatever. No one can have time to develop all those skills.'

Working in Tyson's studio situation caused Titchner to make a significant departure from Tyson's practice. 'It's led me to be more emollient to individual works. Work flying in and out of the studio without being resolved is depressing. I've become more clingy to my practice.'

Mary Horlock, curator of Titchner's Art Now show and author of a forthcoming publication on Julian Opie, says: 'You can see a creative dialogue between Keith Tyson's work and that of Mark Titchner, but you never know whether that's why they were drawn to work together in the first place - like Julian Opie and Michael Craig-Martin - or whether there's a slight tendency to adapt from each other and that comes out afterwards in the work. It's part of the hidden network that goes on.'

And of course the network can be literally hidden: like Damien Hirst's assistants painting spots all day in a room in Borough. The use of assistants may be philosophically compatible with an artist's approach but it also enables the artist to produce more work in less time. Assistants cost around £7-10 an hour, or £120 a day at the top of the market: employing several full-time is a significant investment, and art is a business like any other.

'In the last 10 years art has become part of the wheel of commodity,' argues Richard Wentworth. 'It's to do with the market: artists are expected to be extremely productive and very repetitive.' Wentworth worked in Henry Moore's studio, yet now prefers not to use assistants himself. 'I found I was inventing tomorrow for them instead of me. The British love to get caught up in the idea of who has created an art work yet have a very personal connection to mass-produced products like their cars. So when you live in a culture that is looking for individuality, when in truth there's very little, that shows up in any argument about art, including the way in which artists use assistants.'

By placing an advertisement in the Guardian - unheard of in the word-of-mouth inner sanctums of the art world - Julian Opie was making explicit and public the impact that assistants can have upon an artist's career. 'It's always frustrated me the way in which the British art world can be very amateur. I want to delve into the world on every level and produce fabulously made work like that of Jeff Koons and Andy Warhol. I couldn't do what I'm doing without full-time assistants. Nor could I do it without a computer. But even without, I'd still be an artist and I'd still make things. That's my nature - just like a fish that grows to the size of the tank, assistants allow us to grow our work yet remain the same.'